In a recent build on a macOS, we recognized that Texinfo needs the
'libintl.h' headers of Gettext. However, Gettext depends on M4, and until
now we had set M4 to depend on Texinfo. Therefore adding Gettext as a
dependency of Texinfo would cause a circular dependency.

On the macOS, we temporarily disabled M4's Texinfo dependency, and the
build went through. I also checked on my GNU/Linux system: temporarily
renamed all Texinfo built files from my system and done a clean build of M4
and it succeeded. To be further safe, I built Maneage from this commit
(where M4 doesn't depend on Texinfo) in a Docker container, and it went
through with no problems. So the current M4 version indeed doesn't need
Texinfo. I think adding Texinfo as a dependency of M4 was a historic issue
from the early days.

In the process, I also cleaned 'basic.mk' a little:

 - A "# Level N" comment was added on top of each group of software that
   can be built in parallel (generally).

 - GNU Nano was moved to the end of the file (to be "Level 6").

 - Some comments were edited in some places.